Aircraft vortex detection system

ABSTRACT

An improved system for detection and measurement of aircraft wing tip vortices over a wide area. An acoustic echo system with transmitter and receiver spaced from each other in a plane perpendicular to the flight path for illuminating the vortex and picking up scattering of the transmitter signal produced by the vortex, using Doppler frequency spectrum analysis with the maximum and minimum frequencies providing a measure of vortex intensity. A plurality of receivers operating with a common transmitter providing coverage of an area of interest along a flight path. A transmitter covering a broad area by a broad beam acoustic transducer or a plurality of narrow beams, and means for identifying a specific zone within the broad area occupied by the vortex being analyzed.

mam States Patent Raiser et all.

[451 May 22, 1973 {75] Inventors: Martin Balser, Covina; Arthur E. Nagy,Los Angeles; Andrew P. Proudian, Chatsworth, all of Calif [73] Assignee:Xonics lnc., Van Nuys, Calif.

[22] Filed: Dec. 23, 1971 [2!] Appl. No.: 211,478

[52] US. Cl. ..340/l R, 340/3 D, 343/5 W [51 ..G0ls 9/66 [58] Field ofSearch ..340/1 R, 3 D; 343/5 W [56] References Cited UNITED STATESPATENTS 3,028,578 4/1962 Stanton ..340/l R IQ MODUL'QTOR 69 i 73\ lQa 75m i I WWW 4 355;; N i I Primary ExaminerRichard A. Farley Attorney- FordW. Harris, Jr., Warren L. Kern, Walton Eugene Tinsley et al.

[57] ABSTRACT An improved system for detection and measurement ofaircraft wing tip vortices over a wide area. An acoustic echo systemwith transmitter and receiver spaced from each other in a planeperpendicular to the flight path for illuminating the vortex and pickingup scattering of the transmitter signal produced by the vortex, usingDoppler frequency spectrum analysis with the maximum and minimumfrequencies providing a measure of vortex intensity. A plurality ofreceivers operating with a common transmitter providing coverage of anarea of interest along a flight path. A transmitter covering a broadarea by a broad beam acoustic transducer or a plurality of narrow beams,and means for identifying a specific zone within the broad area occupiedby the vortex being analyzed.

12 Claims, 10 Drawing Figures AIRCRAFT VORTEX DETECTION SYSTEM Thisinvention relates to a new and improved system for the remote detectionand measurement of vortices produced by aircraft in flight. A pair ofcountercirculating wind funnels or vortices are formed behind the tipsof the wing of an aircraft in flight due to the lift generated by thewing. These vortices are often referred to as the tip vortices orwing-tip vortices.

The vortices shed by large aircraft constitute a hazard to smallertrailing aircraft. Controlled tests have shown that such vorticesproduce large rolling movements in aircraft flying through them atdistances up to five miles behind the generating aircraft. The movementand decay of such vortices are variable, depending on the type ofgenerating craft and on atmospheric conditions, and are not sufficientlypredictable to provide reliable information to the pilot of a trailingaircraft as to when a vortex constitutes a potential threat to hisaircraft so that he may take the proper precautionary action. Theproblem is particularly acute near and around airports where theaircraft traffic density is high and where the low flight altitudes makeloss of control more hazardous. It should be noted that aircraftoperation around an airport with separation maintained so great thatpreceding aircraft vortices can safely be assumed to have dissipated isnot consistent with normal high-density airport operation requirements.

It is therefore highly desirable to have a system that can detect andlocate wing tip vortices and determine their strength, so as to permitavoidance of severe vortices by trailing aircraft vulnerable to them.One system for detecting vortices is disclosed in the copendingapplication Ser. No. 97,765, filed Dec. 14, 1970, now U.S. Pat. No.3,671,927, issued June 20, 1972 and assigned to the same assignee asthis application.

The system of this copending application incorporates an acousticDoppler radar or echo system providing an aircraft vortex remotedetection system which can monitor the air space near and aroundairports and provide information for pilot and/or control towerpersonnel on the location and severity of vortices. The informationprovided by the system on vortex intensity and location can be used inthe control of trailing aircraft and in the study of vortex developmentand structure.

This earlier system employed a single, relatively narrow transmitterbeam, which in conjunction with a number of receiver beams providedcoverage of a sequence of adjacent common scattering zones or resolutioncells. The present invention provides certain improvements in theearlier system that allow an acoustic echo system to cover a large area,rather than the relatively narrow swath obtained in the prior system. Anacoustic system places special requirements on the echo system design ascompared to standard radar systems, inasmuch as the long time of travelof the acoustic signal from transmitter to receiver may lead to acondition of overspread signals. The difference in path length betweenthe longest path and the shortest path between transmitter and receivermay be many hundreds of feet, corresponding to a difference in pathdelay of the order of half a second. The fluctuations in the signalscattered from vortex activity would have a bandwidth typically of oneto several hundred Hertz. The time-bandwidth product is thus very muchgreater than unity, leading to some difficulty in analyzing thescattering medium with conventional radar modulations.

The present invention utilizes the plurality of receivers and analyzersof the system of the copending application in combination with atransmitter acoustic transducer system for directing energy over a broadarea encompassing a substantial length of the beam path of each of thereceivers. The system of the present invention also provides a spatialidentification of the particular zone in the area illuminated by thetransmitter which is occupied by the vortex producing the receiveroutput signal; one embodiment utilizes a plurality of adjacent narrowbeams at the transmitter, and another embodiment utilizes a single broadbeam. Accordingly, it is an object of the invention to provide such anew and improved aircraft vortex detection system. While the termacoustic transducer is used in the specification and claims, it shouldbe noted that the term antenna is sometimes used in referring to anacoustic transducer.

Other objects, advantages, features and results will more fully appearin the course of the following description. The drawings merely show andthe description merely describes preferred embodiments of the presentinvention which are given by way of illustration or example.

In the drawings:

FIG. 1 is a perspective view illustrating a vortex detectioninstallation at an airport;

FIG. 2 is a view illustrating the operation of the system of FIG. 1 andlooking along the flight path as indicated by the arrow 2;

FIG. 3 is a view similar to that of FIG. 2 illustrating a vortexdetection system incorporating a presently preferred embodiment of theinvention;

FIG. 4 is a block diagram of a transmitter suitable for use with thesystem of FIG. 3;

FIG. 5 is a schematic diagram illustrating the transmitter of FIG. 4 ingreater detail;

FIG. 6 is a view similar to that of FIG. 3 illustrating an alternativeembodiment of the invention;

FIG. 7 is a block diagram of a transmitter suitable for use with thesystem of FIG. 6;

FIG. 8 illustrates the pulse output of the transmitter of FIG. 7;

FIG. 9 is a block diagram of a single frequency system such as isillustrated in FIG. 6; and

FIG. 10 is a block diagram of a dual frequency system such as isillustrated in FIG. 6.

Referring to the basic system as shown in FIG. 1, a transmitter 10 and areceiver 11 are positioned at an airport, typically on opposite sides ofa runway 12. An aircraft 13 in flight generates counter-rotatingvortices 14, 15 at the tips of the wing. The transmitter and receiverare operated in the acoustic frequency range to provide an acoustic echosystem, with the transmitter acoustic transducer 18 and the receiverantenna 19 directed to a zone indicated generally at 20.

The transmitter directs a beam 21 of acoustic energy to the zone 20.When there is a vortex in this zone, the energy from the transmitter isscattered and portions thereof, indicated by the beam 22, are picked upby the receiver. The reception of this scattered energy from thetransmitter indicates the existence of a vortex in the zone 20. Theground locations of the transmitter and receiver and the axes of thebeams 21, 22 provide the location for the zone. The receiver output maybe processed to provide a signal indicative of the intensity of thevortex detected in the zone.

Acoustic echo systems typically are operated in the frequency range upto about khz and in the system of the present invention, it is preferredto operate in the range of about 2 to 6 khz. The acoustic echo system inthe vortex detection system is operated as a bistatic Doppler system.

The basic mode of operation of the acoustic Doppler vortex detectionsystem is therefore to transmit an acoustic wave of frequency f and todetect the presence of a vortex in the beam path by detecting theacoustic wave scattered from the vortex by the velocity fluctuationswithin it. One measure of the intensity of the vortex can be determinedfrom the Doppler frequency spectrum of the scattered signal, as follows:The velocity v corresponding to a given Doppler shift frequency f,, isgiven by the relation fl 2f(v/c) sin (0/2 where f transmitted frequency,

v velocity of scattering element in the direction u bisecting the anglebetween the incident and scattered wave vectors,

0 speed of sound in air, and

0 scattering angle (see FIG. 2).

The preferred configuration of the system is one in which the incidentwave vector (as determined by the transmitted beam 21 direction) and thescattered wave vector (determined by the receiver beam 22 direction) aresuch that the direction u is substantially normal to the vortex axis(i.e., to the generating aircraft flight path), so that the Dopplerspectrum of the received signal is affected primarily by the tangentialwind veloci ties in the vortex. This may be achieved by positioning thetransmitter and receiver so that the transmitter, the receiver and thezone of interest lie in a plane generally perpendicular to the aircraftflight path. The maximum (positive and negative) Doppler shifts of thescattered signal will then be determined primarily by the maximumtangential velocity v,, present in the vortex. The system of FIG. 3incorporates the multiple beam receiver configuration of the copendingapplica tion in combination with a multiple beam transmitter. In thepreferred embodiment, a phased array of acoustic transducer elements isused to produce the individual narrow beams, each of which is equivalentto a single transmitter beam such as the beam 21 of FIG. 2. In theembodiment of FIG. 3, the transmitter provides beams 21 and 25-30 andthe receiver provides beams 22 and 31-36. Each area of intersection of atransmitter and a receiver beam provides a specifically located zone,such as the zones 40, 41, 42, 43. The transmitter provides forilluminating each of the zones and the receiver provides for picking upscattering from each of the zones as produced by a vortex in a zone,such as the vortex 114 in the zone provided by the intersection oftransmitter beam 21 and receiver beam 22.

The multiple beams may be achieved by utilizing a separate acoustictransducer for each beam or by utilizing a single acoustic transducerarray with electronic or mechanical scanning.

In a typical system, the transmitting and receiving acoustic transducersconsist of arrays of transmitting and receiving elements, therebypermitting electronic scanning and the simultaneous formation ofmultiple beams. The signal normally is transmitted at one or morefrequencies, and the received signal in each receiver channel is fed toa spectrum analyzer with a noncoherent integrator and suitable displays.Information about the location of the vortex is contained in the knownangular positions of the transmitter and receiver beams. Informationabout the intensity of mean and turbulent winds in the vortex iscontained in the frequency shift and spreading of the received signal.

Referring again to FIG. 3, in the operation of the bistatic acousticecho system, the acoustic beams 21, 25-30 are formed by a transmittingantenna 18. A set of beams, seven in the embodiment illustrated in FIG.3, comprising 22, 31-36, is formed by an array receiving acoustictransducer, indicated at 19, separated from the transmitter by adistance of the order of 300 meters. When a turbulent disturbance suchas the vortex 14 is introduced into one of these zones, such as zone 20,energy is scattered out of the transmitted beam 21 into the receiverbeam 22 intersecting that zone, and is fed into the receiver channelappropriate to that beam. The disturbance, the vortex 14 caused by theairplane 13 that has passed overhead a short time earlier, is shown asdescending through zone 20 along the vortex trajectory 35.

Means are provided for distinguishing the different transmitter beams toprovide the spatial identification of the zone in which a vortex isdetected. In one of the distinguishing or identification means, timesharing is utilized, that is, each transmitter beam is energized insequence with a continuous wave signal for a short period. When anoutput signal is obtained for a particular receiver beam, it is knownwhich transmitter beam is being energized and therefore the zone ofinterest is at the intersection of these specific transmitter andreceiver beams. A typical system would utilize in the order of six to 12transmitter beams. It is normally desirable to maintain the transmitterscanning time to a value of no more than about 2 seconds and also todwell on each beam a sufiiciently long time to achieve the desiredanalysis, usually requiring one-quarter to one-half second.

These time factors place some limitation on the number of beams whichcan be scanned. This problem may be resolved by transmitting on separatetransmitter beams simultaneously at different frequencies. The bandwidthof the vortex return normally requires a separation of a kilohertz or sobetween channels. At the same time it is desirable to have thescattering returns to the various channels of the receiver comparable infrequency and bandwidth, and this usually indicates that the number ofsimultaneous frequencies at the transmitter be not more than two orthree.

A transmitter configuration for selectively energizing a plurality ofadjacent transmitter beams by scanning and operation at two frequenciesf and f is shown in FIG. 4. The output of a signal generator 50 isconnected to a scan switch 51 and the output of a signal generator 52 isconnected to a scan switch 53. The signal generators operate at thebasic transmitting frequencies, typically 5 Khz and 6 Khz. Thetransmitter may utilize a plurality of acoustic transducers, with three54, 55, 56 shown in FIG. 4. The frequency f is sequentially connected toeach of the acoustic transducers through a power amplifier 57 and asumming junction 58 by the scan switch 51. The frequency f is similarlysequentially connected to each of the acoustic transducers via thesumming junctions and power amplifiers by the scan switch 53.

A specific embodiment for the transmitter of FIG. 4 utilizing a phasedarray for the acoustic transducer comprising a plurality of acoustictransducers l-M as the acoustic transducer elements is shown in FIG. 5.

The first step in producing the transmitter beams is to generate a setof signals with appropriate time delays or phasing so as to produce aproperly phased set of signals for the radiating elements ortransducers. These are produced by the phased signal generators 50, 52.Scanning is provided by gates 59 under the control of switching logic60, with a gate for each transducer at each frequency. Gain control isprovided for the output of each gate by a potentiometer 61.

The array of the transmitter of FIG. 5 utilizes M transducers to produceN beams. The output T is the properly delayed signal for transducer 1 toproduce transmitter beam 1. Similarly, output T is the properly delayedsignal for transducer 1 to produce beam 2, and so on for each of the Nbeams. Similarly, outputs T -T provide the signals to transducer 2 foreach of the N beams. There are M sets of outputs from a signal generatorfor the M transducers of the array. Dual frequency operation is obtainedby utilizing two generators 50, 52 with corresponding gates and gaincontrol, all under the control of the switching logic 60. The switchinglogic cycles through the proper delays for each successive beam positionby activating the appropriate gates utilizing conventional acoustictransducer scanning techniques. Any type of aperture illumination tapercan be easily obtained by adjusting the gain of each power amplifier.One embodiment in current use incorporates an array of 22 transducerelements, phased to produce four sets of two simultaneous beams atdifferent frequencies, providing a total of eight beams.

Several techniques are available for implementing the phased signalgenerators 50, 52. One consists of a delay line with taps at about 100equal increments, from which the delay appropriate to a given beam and agiven element is selected when required. Use of time delays produces abeam position that is independent of frequency. If the frequency neednot be varied, the delay need only produce the proper phase for thatfrequency, and lines can generally be made shorter and with fewer taps.In this case the maximum delay T l/f, or even T hf if some outputsignals are inverted.

An alternate technique for generating the properly phased signalsutilizes a shift register as a substitute for the delay line. Assumethat the register contains N bits, initially all the same state. Let theregister be shifted at a frequency of 2Nf, with the least significantbit being inverted and fed into the most significant bit when theregister is given the shift command. As a result, a square wave appearsat the Mth register cell, which has a frequency f and a phase 21rM/N, or21rM/N +1:- if the output is inverted. The square wave can then belowpass filtered to produce only the fundamental frequency f, and fedinto its appropriate gate.

A third phasing technique is based on the reciprocity principle. A soundsource is placed in the far field of the acoustic transducer at thedesired angle for each beam. If the acoustic transducer is used as areceiver, signals of the appropriate phase for that direction appear atthe element terminals. These signals are then recorded by an appropriatemeans. With proper editing, beam switching can then be programmed andthe recordings played back through amplifiers to generate the properbeams.

With the system of FIG. 5, the switching logic provides identificationof the transmitter beam being energized at any time for each frequencyfl, f Hence when there is a receiver output indication for one of thefre quencies,f f at a particular time, the particular transmitter beamproducing energy for scattering to the particular receiver beam by thevortex is identified and the known spatial intersection of the two beamslocates the vortex.

A separate receiver channel is provided for each of the receiving beams,and suitable receiver and analyzer channels are shown and described inthe aforesaid copending application. Reference may be made to saidapplication for specific examples of operating characteristics.

An alternative embodiment of the system is shown in FIGS. 6-10. Asimpler approach to area coverage is possible when the source ofscattering is relatively limited in spatial extent. The phased array canbe replaced by a single broadbeam acoustic transducer in conjunctionwith a transmitted signal tailored to the localized vortexcharacteristics. Such a system is shown in FIG. 6 where the transmitterproduces the single broad beam 65 which encompasses a length of the beampath of each of the receiver beams 22, 31-36. A preferred form oftransmitter is shown in FIG. 7 and incorporates a signal generator 66, amodulator 67, a power amplifier 68, and an acoustic transducer 69. Thetransmitter output is pulsed by the modulator and a typical pulsepattern is shown in FIG. 8 with a short pulse of duration T followed bya longer pulse 71, with the short pulse being used for spatialidentification and the long pulse being used for vortex analysis.

The single broad transmitter beam illuminates the entire area to becovered by the system. The short pulse of length T, typically in theorder of 20 milliseconds, is used to localize the vortex in space. If areturn is observed in a particular receiver channel, for example thechannel for beam 34, in the time period T, to T after transmission(where T T T), then the vortex is identified as originating in the zone75. The zone is defined by the intersection of the particular receiverbeam 34 and the ellipsoidal shell with the transmitter and receiver asfoci. This ellipsoidal shell is approximately a horizontal slab for theusual system configuration and thus the elapsed time after transmissionof the pulse 70 may be correlated with altitude above the position ofthe transmitter. In order to avoid spatial ambiguity, the pulse 70should not be transmitted more frequently than once or twice per second.The longer pulse 71 provides the energy for illuminating the vortex toproduce the scattering for the receiver with a pulse duration longenough to permit analysis of the velocity characteristics in the zoneoccupied by the vortex. The analysis is the same as that provided in theearly embodiment and in the copending application.

The longer pulse 71 can be at the same frequency as the short pulse 70and is delayed so as to allow the spatial information to be obtainedfirst.

A system for measuring the time lapse between transmission of the shortpulse 70 and the time an output signal is received from a receiver isshown in FIG. 9. An

elapsed time counter 72 has a start input on line 73 and a stop input online 74. A start signal is provided from the modulator 67 when the shortpulse 70 is generated. A stop signal is provided from one of thereceivers when an output signal indicating the presence of a vortex isreceived. The elapsed time between the two signals is counted and may bedisplayed at an indicator 75 in terms of time or in terms of amplitudeor otherwise as desired.

In an alternative arrangement, the short pulse 70 may be transmittedperiodically at one frequency f and a continuous-wave signal may betransmitted at another frequency f,, with the short pulse being used forspatial analysis and the continuous-wave signal used for velocity orvortex analysis. A typical transmitter configuration for this system isillustrated in FIG. 10, with signal generator 76 providing thecontinuous-wave signal which is combined with the pulse signal frommodulator 67 at a summing junction 77, providing for transmission ofboth frequencies f and f at the same time.

As another alternative mode particularly suitable where very finevelocity resolution is not required and where the signal level isrelatively high, a system as shown in FIG. 9 may be utilized with asingle pulse of about 20 millisecond duration with the return from thispulse being used for both spatial identification and vortex velocityanalysis.

Although exemplary embodiments of the invention have been disclosed anddiscussed, it will be understood that other applications of theinvention are possible and that the embodiments disclosed may besubjected to various changes, modifications and substitutions withoutnecessarily departing from the spirit of the invention.

We claim:

1. In an aircraft vortex detection system incorporating an acoustic echosystem with a transmitter for directing a beam of acoustic energy towarda zone, and a plurality of receivers and analyzers spaced from thetransmitter for receiving acoustic energy of the transmitter scatteredby a vortex in a zone and generating an output signal varying as afunction of the magnitude of the scattering,

the improvement wherein said transmitter includes an acoustic transducersystem with a plurality of beams for directing energy to a plurality ofadjacent beam paths, and control means for selectively energizing saidbeams.

2. A system as defined in claim 1 wherein said control means includesmeans for energizing said beams in sequence.

3. A system as defined in claim 1 wherein said control means includesmeans for energizing two of said beams at different frequencies.

4. A system as defined in claim 1 wherein said control means includesmeans for energizing said beams in groups of different frequencies, andenergizing the beams of a group in sequence.

5. A system as defined in claim 2 wherein said transmitter acoustictransducer system comprises an M element array, and said control meansincludes a phased signal generator providing M sets of transmittersignals, with a gate for connecting each set to an element of the array,and a switching unit for selectively actuating gates to energize saidtransmitter beams in sequence.

6. In an aircraft vortex detection system incorporating an acoustic echosystem with a transmitter for directing a beam of acoustic energy towarda zone, and a plurality of receivers and analyzers spaced from thetransmitter for receiving acoustic energy of the transmitter scatteredby a vortex in a zone and generating an output signal varying as afunction of the magnitude of the scattering,

the improvement wherein said transmitter includes an acoustic transducersystem for directing energy over a broad beam path encompassing a lengthof the beam path of each of said receivers, and control means forenergizing said transmitter beam in pulses, and including timing meanshaving signals corresponding to a transmitted pulse and a received pulseas inputs for providing a spatial identification of the zone occupied bythe vortex producing the received pulse. 7. A system as defined in claim6 wherein said control means includes means for producing alternatelyshort duration and long duration pulses, with said short duration pulsesproviding the transmitted pulse signal to said timing means.

8. A system as defined in claim 6 wherein said control means includesmeans for producing pulses of one frequency and a continuous output ofanother frequency. 9. A system as defined in claim 6 wherein said timingmeans includes means for measuring the time interval between transmittedpulse and received pulse, with said time interval varying as a functionof the location of the vortex in the space illuminated by thetransmitter beam.

10. In an aircraft vortex detection system incorporating an acousticecho system with a transmitter for directing a beam of acoustic energytoward a zone, and a plurality of receivers and analyzers spaced fromthe transmitter for receiving acoustic energy of the transmitterscattered by a vortex in a zone and generating an output signal varyingas a function of the magnitude of the scattering,

the improvement wherein said transmitter includes an acoustic transducersystem for directing energy over a broad area encompassing a length ofthe beam path of each of said receivers, and

identification means for providing a spatial identification of the zonein said area occupied by the vortex producing an output signal at aspecific receiver.

11. A system as defined in claim 10 wherein said acoustic transducersystem provides a plurality of beams defining adjacent beam paths, andsaid identification means includes means for selectively energizing saidtransmitter beams.

12. A system as defined in claim 11 wherein said acoustic transducersystem provides a single broad beam, and

said identification means includes means for energizing said transmitterbeam in pulses and means for measuring the time interval between atransmitted pulse and the received pulse providing the output signal.

1. In an aircraft vortex detection system incorporating an acoustic echosystem with a transmitter for directing a beam of acoustic energy towarda zone, and a plurality of receivers and analyzers spaced from thetransmitter for receiving acoustic energy of the transmitter scatteredby a vortex in a zone and generating an output signal varying as afunction of the magnitude of the scattering, the improvement whereinsaid transmitter includes an acoustic transducer system with a pluralityof beams for directing energy to a plurality of adjacent beam paths, andcontrol means for selectively energizing said beams.
 2. A system asdefined in claim 1 wherein said control means includes means forenergizing said beams in sequence.
 3. A system as defined in claim 1wherein said control means includes means for energizing two of saidbeams at different frequencies.
 4. A system as defined in claim 1wherein said control means includes means for energizing said beams ingroups of different frequencies, and energizing the beams of a group insequence.
 5. A system as defined in claim 2 wherein said transmitteracoustic transducer system comprises an M element array, and saidcontrol means includes a phased signal generator providing M sets oftransmitter signals, with a gate for connecting each set to an elementof the array, and a switching unit for selectively actuating gates toenergize said transmitter beams in sequence.
 6. In an aircraft vortexdetection system incorporating an acoustic echo system with atransmitter for directing a beam of acoustic energy toward a zone, and aplurality of receivers and analyzers spaced from the transmitter forreceiving acoustic energy of the transmitter scattered by a vortex in azone and generating an output signal varying as a function of themagnitude of the scattering, the improvement wherein said transmitterincludes an acoustic transducer system for directing energy over a broadbeam path encompassing a length of the beam path of each of saidreceivers, and control means for energizing said transmitter beam inpulses, and including timing means having signals corresponding to atransmitted pulse and a received pulse as inputs for providing a spatialidentification of the zone occupied by the vortex producing the receivedpulse.
 7. A system as defined in claim 6 wherein said control meansincludes means for producing alternately short duration and longduration pulses, with said short duration pulses providing thetransmitted pulse signal to said timing means.
 8. A system as defined inclaim 6 wherein said control means includes means for producing pulsesof one frequency and a continuous output of another frequency.
 9. Asystem as defined in claim 6 wherein said timing means includes meansfor measuring the time interval between transmitted pulse and receivedpulse, with said time interval varying as a function of the location ofthe vortex in the space illuminated by the transmitter beam.
 10. In anaircraft vortex detection system incorporating an acoustic echo systemwith a transmitter for directing a beam of acoustic energy toward azone, and a plurality of receivers and analyzers spaced from thetransmitter for receiving acoustic energy of the transmitter scatteredby a vortex in a zone and generating an output signal varying as afunction of the magnitude of the scattering, the improvement whereinsaid transmitter includes an acoustic transducer system for directingenergy over a broad area encompassing a length of the beam path of eachof said receivers, and identification means for providing a spatialidentification of the zone in said area occupied by the vortex producingan output signal at a specific receiver.
 11. A system as defined iNclaim 10 wherein said acoustic transducer system provides a plurality ofbeams defining adjacent beam paths, and said identification meansincludes means for selectively energizing said transmitter beams.
 12. Asystem as defined in claim 11 wherein said acoustic transducer systemprovides a single broad beam, and said identification means includesmeans for energizing said transmitter beam in pulses and means formeasuring the time interval between a transmitted pulse and the receivedpulse providing the output signal.